The present invention relates to a surface-acoustic-wave device that can be used in a high-frequency band, and a substrate suitably used for such a surface-acoustic-wave device.
A surface-acoustic-wave (SAW) device utilizing a SAW, which propagates in the surface of a solid body, satisfies properties required for an electromechanical functional part, such as ease in automating and simplifying its mounting operation, since it is small in size and light in weight, is superior in resistance against vibration and impact, and eliminates need for circuit adjustment.
Further, a SAW device has other various advantageous features, such as superior temperature stability, a long life, and superior phase characteristics. Therefore, SAW devices are widely employed as frequency filters, resonators, delay devices, Fourier transformers, convolvers, correlators, optoelectronic functional devices, etc.
Meanwhile, with a recent tendency toward multiple channels and higher frequency in the communication field including satellite communications and mobile communications, there has increased a demand for development of SAW devices that can be used in a high-frequency band (e.g., in a GHz band). A SAW device having a multilayered structure composed of diamond and piezoelectric layers has come into wide use for applications in such a high-frequency band. A method for manufacturing such a SAW device comprises the steps of forming a diamond film by vapor-phase synthesis on a base film made of a material such as Si, polishing the surface of the diamond film into a smooth surface, and forming a piezoelectric layer and inter-digital transducers (IDTs) on the smooth surface, for example.
As for the piezoelectric layer, attention was given to a layer made of ZnO in the past. Recently, however, piezoelectric layers made of LiNbO3 and LiTaO3 have received attention. The piezoelectric layers made of LiNbO3 and LiTaO3 are superior in chemical stability, such as resistance against acids and alkalis, to those made of ZnO. In particular, LiNbO3 has a higher electro-mechanical coupling coefficient (K2) that is a guideline representing easiness in SAW excitation, and is suitably used in a wide-band filter for cellular phones, etc.
It has, however, been difficult to obtain good characteristics with a SAW device that is manufactured simply by forming a layer of LiNbO3 or LiTaO3 on a diamond layer formed by vapor-phase synthesis. Japanese Unexamined Patent Application Publication No. 8-154033 discloses a SAW device that has overcome the above problem. The SAW device disclosed in the Publication has been made based on the finding that the above-mentioned problem was attributable to improper crystal characteristics of LiNbO3. The disclosed SAW device has, between a diamond layer and a LiNbO3 layer, an intermediate layer made of ZnO, Al2O3, MgO or the like. The intermediate layer has the function of controlling crystal characteristics (such as crystallinity and crystal direction) of the LiNbO3 layer so that the characteristics of the SAW device are improved.
However, a SAW device including a piezoelectric layer made of LiNbO3 or LiTaO3 has the following shortcoming. In the SAW device disclosed in Japanese Unexamined Patent Application Publication No. 8-154033, since the intermediate layer between the diamond layer and the LiNbO3 layer is formed of an oxide such as ZnO or MgO, the intermediate layer and the diamond layer are apt to separate from each other.
The present invention has been made with the view of solving such problem, and its object is to provide a substrate for a surface-acoustic-wave device in which a diamond layer and an intermediate layer for controlling crystal characteristics of a piezoelectric layer do not separate easily from each other, and a surface-acoustic-wave device using such substrate.
As a result of conducting intensive studies to achieve the above object, the inventors have found that using AlN as an intermediate layer is very effective in increasing adhesion between a diamond layer and the intermediate layer.
Therefore, a substrate for a surface-acoustic-wave device according to the present invention is featured in comprising a diamond layer, an intermediate layer disposed on the diamond layer, and a piezoelectric layer disposed on the intermediate layer, and a SiO2 layer disposed on the piezoelectric layer, the piezoelectric layer being made of LiNbO3, the intermediate layer being made of AlN.
By so disposing the SiO2 layer on the piezoelectric layer, it is possible to stabilize temperature characteristics of the surface-acoustic-wave device and to prevent a variation of center frequency. This is because the temperature coefficient of SiO2 has a sign opposite to signs of the temperature coefficients of LiNbO3 and diamond, and a canceling effect results.
Preferably, the intermediate layer is made of AlN having C-axis orientation. Since the piezoelectric layer located on the intermediate layer has a tendency to follow the crystal characteristics of the intermediate layer, the piezoelectric layer can easily be made to have C-axis orientation by causing the intermediate layer to have C-axis orientation.
Preferably, the intermediate layer has a thickness of 5 to 100 nm. The reason is that if the intermediate layer is thinner than the lower limit of the above range, the function of controlling the crystal characteristics of the piezoelectric layer tends to be insufficient, and if the intermediate layer is thicker than the upper limit of the above range, the intermediate layer is more apt to impede the characteristics of the underlying diamond layer.
Preferably, the piezoelectric layer is made of LiNbO3 having C-axis orientation. By forming the piezoelectric layer of LiNbO3 having C-axis orientation, piezoelectric characteristics can be improved.
Further, a surface-acoustic-wave device according to an embodiment of the present invention comprises the above-mentioned substrate for a surface-acoustic-wave device and exciting electrodes for exciting a surface acoustic wave.
In the surface-acoustic-wave device according to an embodiment of the present invention, since the intermediate layer is made of AlN as described above, and accordingly higher adhesion is achieved between the intermediate layer and the diamond layer in the substrate of the surface-acoustic-wave device, the resistance against vibration and impact can be improved.
In addition, the inventors have also found that using AlN as the intermediate layer is very effective in improving adhesion between the diamond layer and the intermediate layer in the case of forming the piezoelectric layer of LiTaO3 as with the case of using LiNbO3.
That is, a substrate for a surface-acoustic-wave device according to another embodiment of the present invention comprises a diamond layer, an intermediate layer disposed on the diamond layer, and a piezoelectric layer disposed on the intermediate layer, and a SiO2 layer disposed on the piezoelectric layer, the piezoelectric layer being made of LiTaO3, the intermediate layer being made of AlN.
By so disposing the SiO2 layer on the piezoelectric layer, it is possible to stabilize temperature characteristics of the surface-acoustic-wave device and to prevent a variation of center frequency. This is because the temperature coefficient of SiO2 has a sign opposite to the signs of the temperature coefficients of LiTaO3 and diamond, and a canceling effect results.
Preferably, the intermediate layer is made of AlN having C-axis orientation. Since the piezoelectric layer located on the intermediate layer has a tendency to follow the crystal characteristics of the intermediate layer, the piezoelectric layer can easily be made to have C-axis orientation by causing the intermediate layer to have C-axis orientation.
Preferably, the intermediate layer has a thickness of 5 to 100 nm. The reason is that if the intermediate layer is thinner than the lower limit of the above range, the function of controlling the crystal characteristics of the piezoelectric layer tends to be insufficient, and if the intermediate layer is thicker than the upper limit of the above range, the intermediate layer is more apt to impede the characteristics of the underlying diamond layer.
Preferably, the piezoelectric layer is made of LiTaO3 having C-axis orientation. By forming the piezoelectric layer of LiTaO3 having C-axis orientation, piezoelectric characteristics can be improved.
Further, a surface-acoustic-wave device according to another embodiment of the present invention comprises the above-mentioned substrate for a surface-acoustic-wave device and exciting electrodes for exciting a surface acoustic wave.
In the surface-acoustic-wave device according to another embodiment of the present invention, since the intermediate layer is made of AlN as described above, and accordingly higher adhesion is achieved between the intermediate layer and the diamond layer in the substrate of the surface-acoustic-wave device, the resistance against vibration and impact can be improved.